SUMMARY OF WORK: Lactate dehydrogenases isozymes, purified from mouse muscle (M), mouse heart (H), mouse testis (X), have been cleaved into small peptides by CNBr and trypsin. Most of these peptides have been separated and their amino acid sequences determined. Thus far, 95% of mouse LDH-X, 50% of mouse LDH-M, 10% of mouse LDH-H and 20% of rat LDH-X (330 amino acids in each protein) have been sequenced.